The present invention relates to a method of making fine powder, and to an apparatus for making fine powder, for practicing the method. In particular, the present invention relates to a method of making fine powder and an apparatus for making fine powder in which at least one metal and another element which are to form the powder are rapidly cooled while being combined together by rapid expansion through a convergent-divergent nozzle.
In the following, the expression "compound" will be used in a wide sense, to mean a combination between two (or more) elemental substances, which are either chemically combined together to form a chemical compound, or which are intimately mixed together, as in an alloy. In other words, the expression "compound" will be used to include the possibilities of a chemical compound, an alloy, a solid solution, or a mixture, in a general sense. Similarly, the expression "metallic compound" will be used to mean such a compound, one at least of the elements of which is a metal.
A metallic compound, generally in the prior art, is made from a molten metal and another typically gaseous element, which are combined together to form the compound. This conventionally known method of making such a metallic compound has the disadvantages that: (a) the mixing in of impurities in the metallic compound is unavoidable; (b) it is not practicable to obtain the metallic compound as a fine powder with particle diameters of a few microns or less; (c) since the rate of cooling is limited to about 10.sup.5 .degree. C./sec, when rapid cooling of the molten material is to be performed it is not possible to obtain a metallic compound with good amorphous qualities.
There is also a family of conventionally known methods of making single crystals and thin films of various materials, including the gas phase growth method and/or the vacuum vapor deposition method. However, all of these methods involve gradual vapor deposition on a base plate, and therefore they are all subject to the disadvantages that the metallic material obtained is a mixture consisting of an amorphous layer directly deposited on the base plate and a crystalline layer formed thereover, and that the speed of growth is so slow that it is quite unsuitable for mass production.
In view of the shortcomings of the above described methods, there has been developed another method of making, in particular, silicon nitride fine powder, which is widely used as a raw material for ceramics. In this method, first metallic silicon is formed into a powder, and then this powder is heated within a flow of nitrogen or ammonia gas at a temperature of around 1500.degree. C. or somewhat less, while controlling the pressure of the gas, so that the silicon and the nitrogen react together. However, this method has the shortcomings that the resulting silicon nitride is a mixture of alpha type silicon nitride and beta type silicon nitride, that it is difficult to obtain sufficiently fine silicon nitride powder, and that the metallic silicon which is used as the raw material is required to be pulverized, which takes a long time and is very troublesome.
Yet another prior art method that has been developed for producing silicon nitride involves a gaseous phase reaction between silicon halide and ammonia (or a chlorine compound containing nitrogen and ammonia) at a high temperature of 1000.degree. to 1500.degree. C. However, this method has the shortcoming that post processing is required afterwards, and pulverization and fractionation are required in order to obtain fine powder with a uniform particle diameter of one micron or less, since the raw silicon nitride powder produced is a mixture of particles of various diameters up to about three microns. Another problem with this method is that chlorine or hydrochloric acid which is also produced in the gaseous phase reaction cannot be prevented from becoming mixed with the silicon nitride and contaminating it.